Saw-CTD parallel to serial imager

ABSTRACT

A line imager comprising: a semiconductor body; a planar, transparent piezoelectric body having a main surface overlying and in proximity to the semiconductor body; wave propagation means for propagating acoustic waves on the main surface of the piezoelectric body to create traveling potential wells in the underlying semiconductor body; a traveling potential well path located in the semiconductor, the traveling potential well path beginning at the wave propogation means and extending straight away therefrom; semiconductor depletion means for depleting the semiconductor of majority charge carriers along the traveling potential well path, the depletion means located atop the piezoelectric body; a gate located on the semiconductor body and alongside and parallel to the traveling potential well path and adjoining the semiconductor depletion means; a plurality of sensor pixels for accumulating charge, the sensor pixels located in the semiconductor body, the pixels aligned next to each other and running parallel to, alongside and overlapping the gate so the integrated charge in each sensor pixel will proceed into their respective traveling potential wells when the potential across the gate is lowered, the gate having length at least equal to the length of the plurality of sensor pixels; and a dump gate located on the semiconductor and parallel to and overlapping the plurality of sensor pixels, the plurality of sensor pixels being situated between the dump gate and the gate, the dump gate causing the charge integrated in the plurality of sensor pixels to flow to ground when the potential across the dump gate is lowered, the dump gate being at least as long as the length of the plurality of sensor pixels.

BACKGROUND OF THE INVENTION

The present invention relates to a system for recording electricalwaveforms and more particularly to such a system employing a chargetransfer device.

The high speed recording of photon images involves both mature anddeveloping camera technologies which span the electromagnetic spectrumfrom infrared to x-rays, with temporal resolutions of micro- tofemto-seconds. Often, the cameras, like streak cameras, are veryexpensive, bulky or involve precision machinery, such as rotatingmirrors.

For many applications, it would be advantageous to have small, solidstate imaging devices manufactured and replicated by the methods of thesemiconductor circuit industry. A solid state imager necessarilyincludes three elements: (1) an input comprised of an array of imagesensors, to transduce the incident photon energy into charge carriers inthe device; (2) a charge transfer technique, to move the charges frominput to an output in an organized fashion; and (3) the output, whichmay involve storage, display, or further processing. The arrival ofalternate technologies for any of these three elements can enable thedevelopment of a new class of imagers. In particular, alternatetechnologies for solid state charge transfer devices (CTDs) are nowavailable.

The first CTD, originally conceived by N. Wiener (Cybernetics, JohnWiley and Sons, Inc., New York, 1948, p.144), was the Bucket BrigadeDevice (BBD), in which charge is stored in a serial array of capacitorsand transferred by proper activation of switches placed between them.Several versions of the BBD were later developed utilizing mechanicalswitches, vacuum tubes, or bipolar transistors as the switchingelements. The fully integrated circuit version came about in 1970, whenthe switches were replaced by MOS transistors. See F.L.J. Sangster,"Integrated Bucket Brigade Delay Line Using MOS Tetrodes," PhillipsTech. Review 31;266.

It was in 1970 when W. S. Boyle and G. E. Smith, "Charge CoupledSemiconductor Devices," Bell Syst. Tech. Jour. 49,1970, pp. 587-593,conceived and demonstrated the first Charge Coupled Device (CCD). In itsoriginal version the CCD consisted of closely spaced electrodes on anisolated surface of a semiconductor. With a proper sequence of pulsesapplied to these electrodes, packets of minority carriers weretransferred along the surface of the semiconductor. The impact of theCCD was immense and swift. Almost immediately dozens of schemes andvariations of the device were under study and many potentialapplications were in the making.

Charge Coupled Devices play an important role as self scanned lightsensors. See C. H. Sequin, "Image Sensors Using Surface Charge CoupledDevices," in Solid State Imaging, edited by P. G. Gespers, F. Van derWiele and M. H. White, Noordhoff-Leyden, 1976, pp. 305-329. Sincesilicon is a photosensitive material, charge packets may be created by alight pattern directed onto the surface of the semiconductor. Thesepackets are transported to the output port of the CCD and read outsequentially. The CCD can also perform analog signal processingfunctions such as high density storage (e.g., of an image), analog delayand transversal filtering.

Initial interest in the CCD was heightened by the striking simplicity ofits structure and the range of potential applications. However, as theCCD technology progressed the device performance improved on the onehand while its complexity increased on the other hand. In practicaldevices the electrodes are driven by multiphase clocks (up to fourphases) requiring complex peripheral circuitry. Multilevel electrodesand buried channels are required to improve the efficiency and speed.For imagers, the clock busses occupy a large fraction of the availablearea. These drawbacks limit the speed, density and simplicity of thedevice.

U.S. Pat. No. 4,389,590; "System for recording waveforms using spatialdispersion", R. R. Whitlock (1983) discloses surface acouticcharge-transfer devices that allow for the recordation of electricalwaveforms, be it from an electrical source or a photon source, andallows for bandwidths of greater than 1 GHZ. However, one majorlimitation of this device is that the transfer time across a gate is nofaster than one period of the surface acoustic wave. Another majorlimitation is the requirement for two surface acoustic waves.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to record opticalimages.

Another object of the present invention is to record optical imagesusing a charge transfer device.

These and other objects of the present invention are realized by a lineimager comprising a semiconductor body; a planar, transparentpiezoelectric body having a main surface overlying and in proximity tothe semiconductor body; wave propagation means for propagating acousticwaves on the main surface of the piezoelectric body to create travelingpotential wells in the underlying semiconductor body; a travelingpotential well path located in the semiconductor, the travelingpotential well path beginning at the wave propogation means andextending straight array therefrom; semiconductor depletion means fordepleting the semiconductor of majority charge carriers along thetraveling potential well path, the depletion means located atop thepiezoelectric body; a gate located on the semiconductor body andalongside and parallel to the traveling potential well path andadjoining the semiconductor depletion means; a plurality of sensorpixels for acumulating charge, the sensor pixels located in thesemiconductor body, the pixels aligned next to each other and runningparallel to, alongside and overlapping the gate so the integrated chargein each sensor pixel will fill the traveling potential wells when thepotential across the gate is lowered, the gate having length at leastequal to the length of the plurality of sensor pixels.

The advantages of this invention include its small size and economy ofcost. Its ease of replacability. The potential for short integrationtimes (high temporal resolution). A tolerence for low transferefficiency at the gates controlling charge transfer into or out of thetraveling potential well path. And, the ability for the tranfer time ofcharge carriers across a gate to be below one period of the SAW.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic drawing of a SAW-CTD line imager.

FIG. 2 is a schematic drawing of a basic SAW device that emphasizes theSAW propogation path.

FIG. 3 is a side view schematic drawing of a basic SAW device thatemphasises the SAW charge transfer function.

FIG. 4 a graph of the charge carrying capacity as a function of peakelectric potential of SAW wells.

FIG. 5 is a graph of the charge carrying capacity as a function of SAWfrequency, for various SAW potentials.

FIG. 6 is a graph of charge carrying capacity for low values of wellpotential.

FIG. 7 is overhead schematic representation of a SAW-CTD line imagerwith a subcycle temporal resolution.

FIG. 8 is an overhead schematic representation of a one image line of aSAW-CTD frame addressed framing camera.

FIG. 9 is a representation of the filling of potential wells with chargecarriers.

FIG. 10 is a graph comparing CCD and SAW devices as a function ofprocessing time versus bandwidth.

FIG. 11 is a graph of the minimum cycle time D_(m) versus the size andnumber of pixels.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, and moreparticularly to FIGS. 1, 2 and 3 thereof, there is schematically shown asurface acoustic wave - charge transfer device (SAW-CTD) line imager 10organized as parallel input-serial output.

A continuously powered interdigital transducer 26 sets up the SAW wells20. Sensor pixels 16, e.g. photodiodes, produce and retain charge 25when illuminated. After an integration time, the transfer gate 22, whichseparates the sensors 16 from the charge transfer (CT) zone 18, isdropped. The travelling SAW potential wells 20 are then laterally filledin parallel by conduction of the charges 25 from the sensors 16. Thetransfer gate 22 must be raised prior to the arrival of a SAW well 20filled by the adjacent sensor 16, or spatial resolution will bedegraded. The charges in the SAW wells are then serially transferred tothe output port 30, which may be a pn junction.

More specifically, the basic structure of a Surface Acoustic WaveCharge-Transfer Device (SAW-CTD), shown in FIGS. 2 and 3 and discussedin greater detail below, is similar to a CCD structure with onedifference that the electrodes and clock busses are now replaced by thesurface acoustic waves. A surface acoustic wave travelling in apiezoelectric medium 27 creates electric potential wells 20 which travelwith the acoustic wave and extend into the nearby semiconductor 14.Minority carriers in the semiconductor may be injected into thesepotential wells and transferred synchronously with the wave, i.e., withthe same velocity as the wave.

FIG. 2 shows monolithic structure 13 consisting of an n+ doped contactlayer 15 on the bottom surface of an n doped silicon wafer 12, athermally grown SiO₂ insulating layer 29, and a sputter-coated ZnOpiezoelectric film 27. The perimeter of the device is covered withacoustic absorber 19 to attenuate reflections at the boundaries. Theinput interdigitated aluminum contacts 26 atop the ZnO transduce rfpower from an input lead into SAWs which are launched on the surface andpropagate in both directions away from the input transducer 26. Anidentical transducer 36 may be used for output tests and diagnosis. Ametal field plate 28, to which a potential is applied, is located on theZnO 27. The field plate serves to deplete the Si of majority carrierswhich otherwise would recombine with the minority carriers beingtransported in the travelling SAW wells, and to increase the depth ofthe travelling potential wells in the silicon. The input and outputdiodes 30 are shown in FIG. 3.

FIG. 3 shows a side view of the device of FIG. 2, but here alsoillustrating the diodes 30 and charge transfer process. The diodes 30are p+ regions on the surface of the n- Si substrate 12, with anelectrical connection 21 for bias and signal. Since ZnO and SiO₂ areoptically transparent, the diodes may be operated as photodiodes. Thecharges 25 are injected into the Si 12 at the input diode 30 and areswept up into a travelling SAW potential well 20 which, at that moment,is at the diode. The well is one of a train of potential wells(illustrated to approximate a sine wave) travelling with the acousticspeed, and transports the captured charges 25 along with it. (Thewavelength of the sine wave, in the actual case, would equal theperiodicity of the fingers of one phase of the two-phase transducer,and, for best temporal resolution, the diode size in the progagationdirection in FIG. 3 would be no larger than the wavelength.) Wellsappearing at the output diode 30 at different times contain differingamounts of charge, in accordance with the input waveform. Upon reachingthe output diode 30, the charges are conducted out of the Si and alongthe output lead 23. The diodes 30 should be placed directly under thetransducers 26,36 to obtain a delay time equal to the SAW input-outputdelay time; for devices used in actual applications, the diodes couldboth be on the same side of the input transducer.

Because of its simplicity and potential for high density and speed, theSAW-CTD concept shows promise for a variety of applications in signalprocesing, memory and solid state imaging.

The first study of surface acoustic waves on a solid was done in 1885 byLord Rayleigh, "On Waves Propagating Along the Plane Surface of anElastic Solid," Proc. Lon. Math. Soc. 17, (1885-86) p.4, whoinvestigated the case of waves at the surface of semi-infinite isotropicmedia. Stoneley, Geophys. Suppl. Monthly Notes Roy. Astron. Soc. 5, 1943p. 343, extended the theory to anisotropic media and later other authorstreated elastic waves in piezoelectric anisotropic semi-infinite media.

Layered media have proven to be of considerable interest with respect topiezoelectric surface acoustic waves because of the ability to derivebenefits from the different materials involved. With the increasingsophistication of piezoelectric surface wave devices, the investigationof surface waves has been fully extended into the realm of anisotropiclayered media. For example, various monolithic acoustoelectric andoptoacoustic devices involving multilayered media have been recentlydeveloped. (Monolithic devices are structurally all one piece, as with asilicon substrate overlayered with a piezoelectric ZnO film, indistinction to "gap-coupled" or "separated media" devices in which thepiezoelectric material is mechanically mounted very close to thesemiconductor.) Piezoelectric overlayers are important in severalapplications such as filters, dispersive delay lines and guided wavestructures.

Under certain crystal symmetry conditions or the assumption of weakpiezoelectric coupling, the solutions describing the propagation ofpiezoelectric waves in a medium are decoupled into two independentmodes: one set of solutions corresponding to transverse displacementscalled the "Love Modes," and a second set of solutions whose particledisplacements are confined to the sagital plane and are referred to as"Rayleigh Modes." (The sagital plane is defined as the planeperpendicular to the surface of the structure and along the direction ofpropagation.)

The crystal symmetry of ZnO (hexagonal, class 6 mm crystal), apiezoelectric material 27, fulfills these conditions. The energy of theRayleigh wave is concentrated at the surface of the medium; the particlemotion and their associated potentials decay exponentially with depth,vanishing within a few SAW wavelengths of the free surface. When thewave equation solutions include exponentially dampened sinusoidal terms,the waves are called "generalized Rayleigh waves."

The most effective and commonly employed technique for the excitation ofsurface acoustic waves on piezoelectric media is by means of the metalinterdigital transducer 26. This transducer consists of two sets ofinter-leaved periodic metal electrodes deposited on the piezoelectricsubstrate 27 (see FIGS. 2 and 3). When an rf voltage waveform is appliedacross the transducer, fringing electric fields are set up between themetal fingers, inducing elastic stresses in the underlying substrate byvirtue of the piezoelectric effect. These stresses propagate in the formof piezoelectric surface acoustic waves (Rayleigh waves) in bothdirections perpendicular to the transducer fingers. When the acousticwave reaches an output transducer 36, as in FIG. 2, it is detected bymeans of the surface potential that accompanies the waves. That is, theinput transducer 26 transforms electromagnetic energy to acoustic energyand the output transducer 36 converts it back to electromagnetic energy.The overall input-to-output efficiency with which these conversionsoccur is directly related to the electromechanical (or piezoelectric)coupling coefficient K² of the material; insertion losses (power dropresulting from inserting the device into a circuit) as low as 12db havebeen reported. See R. Wagers, G. Kino, P. Galle, and D. Winslow, "ZnOAcoustic Transducers Utilizing Gold Substrates," Ultasonics Symp. Proc.1972, pp. 194-7.

The center to center distance d between the electrodes is equal to halfthe acoustic wavelength λ and is related to the resonant angularfrequency W_(o) of the transducer and the surface acoustic wave velocityVa by the expression: ##EQU1## Narrow electrodes imply higherfrequencies. For example, a transducer with 10 micron finger width (andthe same separation) corresponds to a wavelength of 40 microns and afrequency of 100 MHz if the surface wave velocity is assumed to be4.0×10⁵ cm/sec.

Since the phase velocity of the acoustic waves is of the order of 10⁵times slower than that of electromagnetic waves, the effect is to delaythe signal by an amount equal to the acoustic transit time between thetwo transducers. For an acoustic wave with velocity 4.0×10⁵ cm/sec andtransducer separation of 1 cm the corresponding delay would be 2.5microsec. This SAW delay line constitutes the basic structure for manysurface wave devices in signal processing and solid state imaging.

The SAW-CTD may be classified as an acoustoelectric device, since likeother devices belonging to this family it is based on the principle ofacoustoelectric interaction with carriers in a semiconductor. Suchdevices include the acoustoelectric amplifier, the acoustic convolver,and acoustoelectric imagers. However, the SAW-CTD device differs fromthe other acoustoelectric devices in two respects. First, it is aminority charge interaction device, and secondly it is a synchronousinteraction device which means that the carriers are captured andtransferred with a speed equal to that of the surface acoustic wave.

The principle for the operation of the SAW-CTD, in which chargedparticles are accelerated to the wave velocity in a solid, is analogousto that of the Travelling Wave Linear Accelerator (linac). See L.Brillouin, "Waves and Electrons Travelling Together - A ComparisonBetween Travelling Wave Tubes and Linear Accelerators," Physical Review,74/1, July 1948, pp. 90-93, in which charged particles are acceleratedand carried along on the crest of an electromagnetic wave travelling infree space, much like the surfer who rides on a single ocean wave. Thesimplest structure of the linac consists of the cylindrical waveguide,in which different modes of propagation can be excited. However, forcharge transfer along the axis, a longitudinal electric field isrequired. Although a longitudinal travelling electric wave can beproduced with the TM01 mode of the waveguide, such a device is notpractical for particle acceleration in free space since the phasevelocity Vp of the travelling wave always exceeds that which anyparticle can attain, no matter what its energy. As a consequence, thetravelling wave pattern will rush past the particle alternatelyaccelerating and decelerating it, without giving it a commulative energygain. Nevertheless, by sharply corrugating the inside walls of thewaveguides, inductance is added to the equivalent network of the guideand Vp is adjusted along the axis of the waveguide. A packet of chargemay then be injected into the path of the travelling wave and beaccelerated along the axis of the structure.

In the operation of the SAW-CTD, the linac waveguide is now replaced bya transfer channel 18 defined on the surface of a semiconductor 14. Theforce needed for the acceleration and transfer of the charge is providedby a travelling acoustoelectric potential generated by a piezoelectricsurface acoustic wave. The acoustic wave has a velocity slow enough tocapture thermal carriers with velocities opposite to the direction ofpropagation (in marked contrast to the requirements of a linac). WhenSAWs propagate in a piezoelectric layer, they are accompanied byelectric fields which extend beyond the surfaces of the layer. Byplacing such a piezoelectric layer over a semiconductor it is possibleto generate travelling potential wells at or below the semiconductorsurface. In the monolithic devices discussed below, field energynormally occupying free space can be forced below the surface and intothe bulk by means of a metal field plate 28 atop the piezoelectriclayer, thus strengthening the well. The simplest technique for injectingthe signal charge into the travelling wells is by direct electricalinjection of a p-n junction 30: injected charge is proportional to thesignal applied to the input diode. See N. A. Papanicolaou, "A MonolithicSurface Acoustic Wave Charge Transfer Device, " Ph.D. Thesis, ElectricalEngineering Department, University of Maryland (1979). Another method isphotoelectric injection in the silicon: charge injected into a well is afunction of the incident light flux, and the device can thus be used forimage sensing purposes. See R. J. Schwartz, S. D. Gaalema, R. L.Gunshor, "A surface wave interaction charged coupled device," IEEE 1976Ultrasonics Symposium, 1976, pp. 197-200. The charges injected by theinput diode are stored in these propagating wells and transferredsynchronously with the wave. The delayed signal is collected a SAWtransit time later at the output diode. See FIGS. 2 and 3. The fieldelectrode 28 over the propagation region 18, in addition to enhancingthe acoustoelectric potential at the silicon surface, also provides thebias for the depletion of unwanted majority charge carriers from thesemiconductor surface, which is required for minority charge transfer,and prevents lateral migration of the minority charges to outside thetransfer region. The interdigitated output tranducer 36, though notneeded for charge transfer operations, is useful for monitoring andevaluating the surface acoustic wave.

The distribution and quantity of charge carried by a SAW potential wellis largely determined by the parameter A. ##EQU2## where Va =theacoustic speed of the SAW, 2πf =the angular frequency of the SAW, μ_(s)=the surface mobility of the charge carriers in the semiconductor, andφ_(o) =the maximum amplitude at the Si-SiO₂ interface of the electricpotential φ_(a) associated with the acoustic wave. For A <1, chargepackets can be captured in the wells, and the distribution of charges inthe well is as depicted in FIG. 9b. FIG. 9 shows the process of fillingSAW potential wells 20. Potential wells 20 are depicted as spatialdimension (horizontal axis) versus potential (vertical axis). Thefilling of SAW wells 20 with charge carriers is controlled largely bythe parameter A (see Equation 1). (a) High frequencies provide highfields but low charge capacities, due to the decreased size of the well.(b) Low values of A permit a greater capacity of the well withoutspillage of charges into an adjacent well. In the figure, phase angleswith an m subscript refer to maximum charge capacity conditions. Theangular relationships are:

    sin θ.sub.o =-A,                                     (2)

and

    cos θ.sub.1-Aθ.sub.1 =cos θ.sub.o -Aθ.sub.o.(3)

Further analysis yields the result that the maximum charge carryingcapacity Q_(m) of a well is given by: ##EQU3## where w=the width of thewell (perpendicular to SAW propagation), and S=the elastance of thestructure. Since the angles are determined by A, the value of A controlsthe quantity in braces, which is always less than unity. For low A, nearzero, the charge occupies nearly the whole well, whereas for A near butless than unity the charges only occupy the highest field portion of thewell. Increasing W or φ_(o) increases Q_(m), as does decreasing S.Through the value A, increasing f increases Q_(m) (by making theelectric field associated with the well stronger) and increasing Vadecreases Q_(m) (by spilling charges out of the well, e.g. when thecharges are initially injected into and accelerated by the well).However, these effects of f and Va are in competition with theirrelationships to Qm in the leading quantity of Equation 4.

Values of Qm were computed and are given in FIGS. 4-6 for a range ofparameters, under the conditions Va=4.4 ×10⁵ cm/s, w=20 microns,S=4.6×10¹⁷ cm² /Farad and μ_(s) =750 cm² /V-s. The general decrease ofQm for higher frequencies can clearly be identified.

Especially noteworthy is the prediction that the order of 10⁴ electronsmay be transported in a single well of width w=20 micrometers at 1 GHzfrequencies. Noise levels of under 100 electrons/pixel are achievable insolid state devices, and a signal as low as 50 electrons/pixel canrender a discernible pattern. A 1 GHz device would require 1 micrometerwide fingers for the interdigitated transducer, which aretechnologically feasible. Higher frequencies may ulitmately be limitedby acoustic diffraction, e.g. from thousand Angstrom wide grains of ZnO;other materials and device architectures may permit yet higherfrequencies.

In discussing the application of solid state devices to imaging, it isconvenient to set forth some of the terminology presently in use. Astill camera (temporally integrating, with respect to the event ofinterest) is a multichannel recording device (one channel per resolvablepicture element or "pixel"), while a temporally resolving camera is atype of multichannel waveform recorder. A temporally resolving camerawith one dimension of spatial imaging is known as a streak camera, whiletwo dimensions of spatial imaging with temporal resolution result in aframing camera. Framing cameras, like a movie camera, normally recordthe whole 2D image in integral frames, a given frame being exposedduring the same interval over its entire area. Other cameras, like ascanning TV camera, record different times at different locations in theimage, and may be referred to as image dissecting cameras, particularlyif the event of interest is changing on a time scale comparable to orless than the time spread of one frame.

Solid state imaging devices may be built as multichannel arrays ofsensing and recording elements (with a charge transfer region betweenthe sensing elements and recording elements). While many solid statesensors have been developed, designs shall be described herein in termsof the basic structure of FIG. 2, with silicon diodes being the sensorpixels 16 or static wells 41. It is to be understood that the injectordiode 16 itself may be the photon imaging sensor, or the injector diode16 may serve as an electrical input with signal supplied by a detectingsensor located elsewhere. The output elements may also be silicondiodes, or a Metal/Oxide/Semiconductor (MOS) capaciter (if each channelof output represents a single value).

The simplest single channel SAW-CTD is shown in FIG. 3. A photodiode orelectrical charge-injection diode 16 (as the figure is drawn) injectscharge into the SAW train. The charge is transferred to the outputdiode, which collects the charge from the SAW wells and conducts it awayon the output conductor. This device has the input and output in directline along the direction of SAW propagation. The net effect is to delaythe signal between the input and output diodes by the propagation timeof the SAW. Multiple channels of this device could be placed side byside to record a 1D spatial image, but such a device would befunctionally equivalent to a simple line of sensor diodes, except forthe charge transfer and resultant delay.

A line imager is depicted schematically in FIG. 1. In this device, alinear array of sensor pixels 16 integrates charge 25 for a controlledperiod of time during which the charges 25 are retained in the sensordiodes 16. Charge retention is accomplished by the presence of fixedpotential barriers (not drawn) which surround each pixel 16 on threesides and a transfer gate 22 on the remaining side.

These barriers and gates 22 may be physically fabricated in differentways, (See, M. J. Howes and P. U. Morgan eds. "Charge-coupled Devicesand Systems", Wiley Interscience, N.Y., 1979) all having the commoncharacteristic that they modify the lateral potential profile in thesemiconductor to prevent unwanted migration of the charges. Fixedbarriers or "channel stops" may be made by increasing the dopingconcentration at the location of the barrier. A variable barrier, orgate 22, takes the form of an insulating layer 29 on the Si 12 and ametal electrode 28 over top, with a potential applied to the metal; thepotential profile in the Si 12 is architectually determined by thethickness of the insulator 22 (which may be fabricated to differentthickness in a predetermined areal pattern), and may be varied in levelaccording to the voltage applied to the metal electrode.

The transfer gate 22 between the sensing array 17 and the chargetransfer (CT) zone 18 is dropped (by bringing the metal line forming thegate 22 to a potential approximately equal to the potential of the fieldplate), permitting the integrated charge 25 in the sensors 16 to migratelaterally along the SAW wells 20 and into the charge transfer region 18.In general, the sensors 16 are no larger than one SAW wavelength in thedirection of SAW propagation, for highest charge capacity, and to avoidloss of spatial resolution. The speed of the gate is governed by itscapacitative and inductive decay times; GHz clocking of the gate 22 canbe achieved (see Howes, supra.) The transfer gate is then raised toprevent the passing SAW wells 20 from receiving charges from more thanone pixel 16. In actuality, the variation in potential at the edge ofthe biased field plate 28 over the charge transfer zone 18 serves tolaterally define the travelling SAW wells 20 and retain the charges 25.Once in the CT zone 18, the charges 25 are moved to the output diode 30and collected. The temporal organization of the device is parallel input(sensors to SAW-CT zone) and serial output (at the end of the CT zone).In the output waveform, different times represent different pixels, i.e.different spatial locations in the sensor array. The temporal resolutionfor continuously cycling operation is determined by the duration betweensuccessive openings of the transfer gate 22; this duration may not beshorter than the time required for the SAW to travel the length of thesensor array 17, lest a pixel 16 be opened to an occupied SAW well.Temporal resolution may be doubled (gate cycle time halved) by adding asecond transfer gate 33 and CT zone 34 below the sensors 16 in FIG. 1,(see FIG. 7) and opening the two transfer gates 26 alternately; minimumcycle time in each CT zone remains the same.

Minimum cycle duration D_(m) is given in FIG. 11, as a function of n thenumber of pixels in the linear array,

the SAW wavelength, and v_(a) the SAW speed, for a typical value ofv_(a) =4.4×10⁵ cm/s in a ZnO-Si SAW-CTD. The minimum duration D_(m) ofthe cycle time for a multielement SAW charge transfer zone is thetransit time of the SAW train by the elements. D_(m=n) λ/v_(a) where λis the size of the element (pixel or SAW wavelength), n is the number ofelements, and v_(a) is the speed of the acoustic wave.

A direct modification can be made to the device of FIG. 1 to permitsub-cycle resolution. In FIG. 7 a second transfer gate 33 (otherwiseknown as a dump gate) has been added to dump charge 25 from the pixels16 directly to ground 35. Since conduction is much faster than SAWcharge transfer, the dump gate 16 may be used to quickly clear thesensors of accumulated charge and begin a new image integration.Temporal resolution is effectively the delay between the dump gate andthe SAW-CT zone transfer gate. However, the constraint remains that onlyone SAW-CT image may be taken per D_(m).

The line imagers of FIGS. 1 and 7 may be employed as single lineelements of a two dimensional imaging array, to make up a framingcamera. The technique of sub-cycle temporal resolution may also beachieved in the framing camera, but a new cycling constraint appearswhen successive lines of the image are serially output into a datastream. Sequentially reading the lines of the image limits the frequencywith which successive frames may be taken. Shorter resolution times areobtainable by operating in the image dissecting mode: the sub-imagesrecorded by different lines represent the event at different times.

Whole frame operation can remove the constraint imposed on temporalresolution by line-sequencing. FIG. 8 illustrates the principle ofsensing the image with an areal array 17 as discussed above, andtransporting the image by SAW-CT to an image recording array 40 ofstatic potential wells 41. These static potential wells 41 may be MOScapacitors as in a CCD, of size consistent with the SAW wavelength forbest resolution. The charges 25 in the traveling SAW wells 20 areseparated from the static wells 41 by a storage transfer gate 42, whichis dropped at the proper time to permit the travelling charges 25 to bestored in the static wells 41. Once the image is recorded in the MOSarray 40, it may be read out in a serial fashion (CCD readout). Thetemporal resolution of this device is set by the switching and chargetransfer times at the transfer gate 22 and the dump gate 33.

The prospect of using SAW-CTDs for particle detection is not to beoverlooked, whether detection is performed directly as drawn in theabove figures or indirectly with detected electrical signals coming, asin FIG. 3, from particle detecting pixels located away from the SAW CTzone.

The low speed of surface acoustic waves is advantageous in manyapplications, while also imposing some limits on practicality. Forinstance, the device of FIG. 1 is constrained to operating above about10 MHz, for which a 1 cm CT zone yields but 22 wavelengths (outputwells) for Va =4.4×10⁵ cm/s. Thus, SAW devices find application at thehigher frequencies, often significantly higher than typical CCDfrequencies. This is illustrated in FIG. 10 for CCDs and for SAW devicesnot involving charge transfer. FIG. 10 is a comparision of CCDs andSurface Wave Devices (SWDs). The application of SWDs has been tofrequencies as high as 1 GHz, and CCds to lower frequencies, withconsiderable overlap. T_(d) is time delay, and T_(d) w is atime-bandwidth product. Values are approximate, for actual devices otherthan SAW-CTDs, and do not represent theoretical limits. The figuresummarizes devices in use, but it does not represent design upper limitsor portray the fastest reported SAW-CTDs or CCDs, some reaching 1 GHz ormore, which are under development for non-imaging applications. HigherSAW frequencies than those shown in the figure may be attained by takingthe pains necessary to obtain submicron linewidths in transducerfingers, gates, injectors, barriers, etc. For a given linewidth, higheracoustic speeds result in higher frequencies. For faster waveformrecording and imaging, the higher frequencies are essential.

One of the most fundamental limits on charge transfer and speed inSAW-CTDs is the dependence of well capacity on frequency andacoustoelectric potential. The frequency dependence is given in FIGS.4-6 for a particular device structure, and may be summarized briefly asexhibiting a definite peak in charge capacity but falling only veryslowly at high frequencies, with a nearly linear dependence on potentialat the higher frequencies. The maximum achievable potential is set bynumerous factors, including the high power limit and design of the SAWlauncher (number of fingers, frequency, capacitance, shape,piezoelectric coupling coefficient, etc.), the materials constants ofthe constituents (carrier mobility, permittivity, piezoelectricconstants, etc.), and the architecture of the device (boundaryconditions, dimensions, etc.). The requirements to operate the device indepletion, rather than accumulation or inversion, sets a materials limiton φ_(o) ; notice however that inversion layers can be swept clear ofcharges by a travelling SAW train. There are definite design tradeoffsto be evaluated in going to higher frequencies. For instance, at a givenfrequency, the potential has a maximum at a particular thickness of ZnO.A recent study of charge capacity in ZnO-Si SAW-CTDs predicts highercapacities to be achievable with the Sezawa mode of SAW propagationrather than with the lower order Rayleigh mode. Thus, a set ofcalculations such as given in FIGS. 4-6 may not represent the optimizedattainable potential.

FIG. 4 is a graph of charge carring capacity as a function of peakelectric potential of SAW wells, for a single 20 micron wide SAW well,under the conditions v_(a) =4.4×10⁵ cm/s, w=20 microns, S=4.6×10¹⁷ cm²/F, and μ_(s) =750 cm² /V-s for electron carriers (hole mobility is 300cm² /V-s). Charge is given in coulombs and in number of electrons, forvarious SAW frequencies.

FIG. 5 is a graph of charge carrying capacity as a function of SAWfrequency, for various SAW potentials. Note the existence of a peak incapacity.

FIG. 6 is a graph of charge carrying capacity for low values of wellpotential. Even for low potentials, GHz frequencies, and narrow wells,significant numbers of electrons may be transported in a single welloriginating from a single pixel.

In addition to charge capacity, charge transfer efficiency (CTE) mustalso be considered. The transfer of charge in the present SAW-CTDdevices occurs in several different operations: charge injection from anin-line diode 16 into the SAW wells, transfer from a sensor pixelthrough a gate and laterally along the trough of a SAW well to fill thewell, charge transport synchronously with the SAW velocity in the SAWwells (in the SAW propagation direction), charge extraction from the SAWwells by an in-line output diode 36, lateral transfer of charge alongthe trough of a SAW well through a transfer gate 22 and into a staticwell 24, and the readout of the static (CCD) wells 24. For each of thesecharge transfer operations there is a corresponding CTE: injection CTE,gate CTE, lateral CTE, synchronous CTE, extraction CTE, storage CTE, andCCD CTE. Some of these are discussed briefly below.

High speed charge transfer in certain types of CCDs (i.e., surfacechannel CCDs) is limited by inefficiencies introduced at the Si-SiO₂interface by fast-acting interface states. These states act as carriertraps which hold and later release charge. The effect of interface orsurface states on SAW-CTDs has been experimentally and theoreticallyinvestigated. These traps may be dealt with in three ways: by keepingthem filled (the "fat zero" in CCD jargon), by avoiding them (the CCDburied channel), or perhaps by operating at speeds faster than or muchslower than the states.

Firstly, it would be easy to implement the fat zero into a SAW CT zone:a diode, placed in the CT zone 18 prior to the signal injecting diode,could bleed a steady state current into the SAW wells sufficient to keepmost of the interface states filled. None of the wells would be leftcompletely empty, and zero signal level would be represented by a finitebut small quantity of charge called the fat zero. The fat zero approach,however, is not a complete solution to the problem. Since the deepestportion of the well fills first, nearly empty wells (the fat zero)occupy less of the well than wells full of signal charges; thus, thedwell time of the fat zero at the traps is less than the signal.Alternatively, the CT zone 18 could be photo-illuminated at a lowirradiance. Secondly, buried channels in CCDs are a micrometer or morebeneath the interface. For surface acoustic waves, the acousticpotential decays exponentially to nearly zero at a depth of a fewwavelengths below the top surface of the ZnO, in the absence of a fieldplate 28. Nonetheless, a buried channel ZnO-Si SAW-CTD may be possibleto construct, since the presence of the field plate 28 above the ZnO, asin FIGS. 2 and 3, alleviates this situation dramatically by increasingthe potential at the SiO₂ -Si interface and by decreasing the rate ofdecay with depth. In a GaAs SAW-CTD, on the other hand, a buried channeldevice has been demonstrated. And thirdly, as for outrunning the traps,this is done already with the very slow traps (release times of days).The crucial quantity in question is therefor the number density of trapslocated in the CT channel 18 with filling and releasing rates matched tothe transit time of the signal charge past the trap. Since this numberis energy dependent, analyzing the situation becomes more complicated.In addition, the traps may behave as scattering centers and therebydecrease mobility. Buried channels reduce the effectiveness of the trapsby transferring the charges through a spatial minimum (or at least areduced value) in the number distribution function of traps; there isalso the possibility of transferring charges through a temporal minimumor energy minimum in the number distribution of traps as a function oftrapping and releasing rate and cross section. SAW-CTDs may actually beof use in measuring these quantities, since charge capacity is dependenton mobility, energy varies with impressed potential, and temporalresolution is a function of trapping and releasing rates and crosssections. As a general rule, however, note that higher frequency SAWshave shorter wavelengths, shorter travel distances for a given number ofpixels, and therefore intercept fewer surface traps.

Lateral charge transfer bears similarity to CCD charge transfer. Chargetransfer through an open gate in CCDs proceeds by three mechanisms,self-induced fields, fringing fields from the gate electrodes, andthermal diffusion of the charge carriers. Self-induced fields, resultingfrom the electrostatic repulsion of charges in the wells, is thedominant impetus when the charge density in the well is high, e.g. whenthe well is more than about 10% occupied. For fewer charges in the well,fringing fields and thermal diffusion, a slow process, predominate. InCCD's very high transfer efficiencies are required. For an overallefficiency of 90%, a 100-stage three-phase CCD must pass a charge packetthrough hundreds of gates, each having a single transfer efficiency of99.97%. This imposes a severe frequency limitation on CCDs.Nevertheless, GaAs CCDs have reportedly achieved above 1 GHz operation.

Although the SAW-CTDs do have a fast transfer gate 22 for transferringcharge carriers into or out of the CT zone, which is analogous to a CCDgate, there is only one or two such gates for image acquisition in thedevices discussed here. Charge carriers remaining in the input sensorsafter the sensor transfer gate is raised can be removed through the dumpgate. Charge carriers remaining in the CT zone after the storagetransfer gate is raised are discarded and do not become a source ofnoise. Thus, considerable transfer inefficiency can be tolerated withoutsmearing charges between adjacent wells. Therefore, higher gate transferfrequencies should be attainable by SAW-CTDs.

The device architectures disclosed herein may also be applied togap-coupled devices or monolithic GaAs, where the GaAs is used in placeof the monicithic structure 13.

Finally, it should be noted that electrical leads and connections run toand from the various elements of the invention but have been omitted.

Obviously, numerous (additional) modifications and variations of thepresent invention are possible in light of the above teachings. It istherefore to be understood that within the scope of the appended claims,the invention may be practiced otherwise than as specifically describedherein.

What is claimed and desired to be secured by Letters Patent of theUnited States is:
 1. A line imager comprising:a semiconductor body; aplanar, transparent piezoelectric body having a main surface overlyingand in proximity to the semiconductor body; wave propagation means forpropagating acoustic waves on the main surface of the piezoelectric bodyto create traveling potential wells in the underlying semiconductorbody; a traveling potential well path located in the semiconductor, thetraveling potential well path beginning at the wave propogation meansand extending straight away therefrom; semiconductor depletion means fordepleting the semiconductor of majority charge carriers along thetraveling potential well path, the depletion means located atop thepiezoelectric body; a gate located on the semiconductor body andalongside and parallel to the traveling potential well path andadjoining the semiconductor depletion means; a plurality of sensorpixels for accumulating charge, the sensor pixels located in thesemiconductor body, the pixels aligned next to each other and runningparallel to, alongside and overlapping the gate so the integrated chargein each sensor pixel will proceed into their respective travelingpotential wells when the potential across the gate is lowered, the gatehaving length at least equal to the length of the plurality of sensorpixels; and a dump gate located on the semiconductor and parallel to andoverlapping the plurality of sensor pixels, the plurality of sensorpixels being situated between the dump gate and the gate, the dump gatecausing the charge integrated in the plurality of sensor pixels to flowto ground when the potential across the dump gate is lowered, the dumpgate being at least as long as the length of the plurality of sensorpixels.
 2. A line imager as described in claim 1 which includes astorage transfer gate located on the semiconductor body and alongsideand parallel to the traveling potential well path and adjoining thesemiconductor depletion means,a plurality of static wells located in thesemiconductor body aligned next to each other and running parallel to,alongside and adjoining the storage transfer gate, the plurality ofstatic wells situated downstream of the plurality of sensor pixels withregard to the traveling potential wells and beginning after theplurality of sensor pixels and the gate have ended, the storage transfergate being at least as long as the plurality of static wells; andbeginning after the gate has ended.
 3. A waveform recorder as describedin claim 1 wherein the semiconductor body is comprised of a planarinsulating body upon which the piezoelectric body is overlying and inproximity therewith; an N doped silicon wafer underneath the planarinsulating body; and an N+ doped contact layer underneath the N dopedsilicon;and wherein the piezoelectric body is a sputter-coated ZnOpiezoelectric film.
 4. A waveform recorder as described in claim 1wherein the wave propagation means is a metal interdigital transducer.5. A waveform recorder as described in claim 1 wherein the sensor pixelsare photodiodes.
 6. A waveform recorder as described in claim 1 whereinthe semiconductor depletion means is a conductive plate.
 7. A waveformrecorder as described in claim 2 wheein the static wells are MOScapacitors.
 8. A waveform recorder as described in claim 2 wherein thegates are made of metal.
 9. A line imager as described in claim 1wherein the sensor pixels are P-N junctions, and including remote sensorpixels and electrical connections linking the remote sensor pixels tothe P-N junctions, one remote sensor pixel being linked to only one P-Njunction.
 10. A line imager comprising:a semiconductor body having aplanar insulating body, an N doped silicon wafer underneath the planarinsulating body, and an N+doped contact layer underneath the N dopedsilicon; a planar piezoelectric body made of sputter coated ZnO having amain surface overlying and in proximity to the planar insulating body ofthe semiconductor body; a metal interdigital transducer for propagatingacoustic waves on the main surface of the piezo-electric body to createtraveling potential wells in the underlying semiconductor body; atraveling potential well path located in the semiconductor body thetraveling potential well path beginning at the wave propagation meansand extending straight away therefrom; a conducting plate for depletingthe semiconductor of majority charge carriers along the travelingpotential well path, the metal plate located atop the piezoelectricbody; a gate made of metal located on the semiconductor body andalongside and parallel to the traveling potential well path andadjoining the metal plate; a plurality of photodiodes for accumulatingcharge, the photodiodes located in the semiconductor body, thephotodiodes aligned next to each other and running parallel to,alongside and overlapping the gate so the intergrated charge in eachphotodiode will proceed into their respective traveling potential wellswhen the potential across the gate is lowered, the gate having length atleast equal to the length of the plurality of photodiodes; a metal dumpgate located on the semiconductor and parallel to and overlapping theplurality of photodiodes, the plurality of photodiodes being situatedbetween the dump gate and the gate, the dump gate causing the chargeintegrated in the plurality of photodiodes to flow to ground when thepotential across the dump gate is lowered, the dump gate being at leastas long as the length of the plurality of photodiodes; and a pluralityof MOS capacitors located in the semiconductor body aligned next to eachother and running parallel to, alongside and adjoining the storagetransfer gate, the plurality of static wells situated downstream of theplurality of photodiodes with regard to the traveling potential wellsand beginning after the plurality of photodiodes and the gate haveended, the storage transfer gate being at least as long as the pluralityof MOS capacitors and beginning after the gate has ended.
 11. A methodfor recording optical images comprising the steps of:integrating chargeat a series of discrete locations, the charge formed by excitations,which correspond to the image to be recorded, at the discrete locations;releasing the integrated charge in the series of discrete locations intotraveling potential wells, each discrete location transferring chargeinto one traveling potential well, each traveling potential wellreceiving charge from only one discrete location; dumping charge toground from the discrete locations so charge undersired to be integratedis removed from the discrete locations and the discrete locations arereadied for another integrating step; transferring charge in thetraveling potential wells into static wells, each static wells receivingcharge from only one traveling potential well; and reading out themegnitude of charge in each static well.